U.S. patent number 5,059,582 [Application Number 07/326,918] was granted by the patent office on 1991-10-22 for superconductor-metal laminates and method of making.
This patent grant is currently assigned to The Research Foundation of State University of NY. Invention is credited to Deborah D. L. Chung.
United States Patent |
5,059,582 |
Chung |
October 22, 1991 |
Superconductor-metal laminates and method of making
Abstract
A superconducting laminate having at least one layer of metal
and at least one layer of superconducting material. The metal layer
and the superconducting layer are bonded. The metal later may also
include carbon fibers from various precursors. The superconductor
may be a composite material. The invention also includes a method
of making the laminates.
Inventors: |
Chung; Deborah D. L.
(Pittsburgh, PA) |
Assignee: |
The Research Foundation of State
University of NY (Albany, NY)
|
Family
ID: |
23274312 |
Appl.
No.: |
07/326,918 |
Filed: |
March 22, 1989 |
Current U.S.
Class: |
505/230; 428/323;
428/457; 428/688; 505/701; 505/703; 428/408; 428/930; 505/702;
505/704; 505/233; 505/433; 505/236 |
Current CPC
Class: |
H01L
39/248 (20130101); H01L 39/143 (20130101); Y10T
428/30 (20150115); Y10S 505/702 (20130101); Y10S
505/701 (20130101); Y10T 428/25 (20150115); Y10S
505/704 (20130101); Y10S 428/93 (20130101); Y10S
505/703 (20130101); Y10T 428/31678 (20150401) |
Current International
Class: |
H01L
39/24 (20060101); B32B 009/00 () |
Field of
Search: |
;505/1,701-704
;428/688,930,457,408,323 ;29/599 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
IEEE Transaction on Magnetics, vol. May 17, 1981, Spenor et al.,
pp. 1006-1009. .
E. I. Monthly, Carbon Fiber Reinforced Zin-SC composites, Journ
Matls Research, vol. 4, No. 6, 11/12/89, pp. 1339-1346..
|
Primary Examiner: Ryan; Patrick J.
Attorney, Agent or Firm: Dunn; Michael L. Mudd; James F.
Park; Ellen K.
Claims
What is claimed is:
1. A superconducting laminate comprising first and second layers;
said first layer comprising a high T.sub.c ceramic superconductor
and said second layer comprising a metal; said first layer being
diffusion bonded to said second layer, wherein said metal is
selected from the group consisting of tin, indium and tin lead
alloy.
2. The laminate as recited in claim 1 further comprising a third
layer, said third layer being adjacent to and being diffusion
bonded to a second side of said superconducting layer such that the
first layer is adjacent to the first side of said superconducting
layer.
3. A superconducting laminate comprising first and second layers;
said first layer comprising a high T.sub.c ceramic superconductor
and said second layer comprising a metal and carbon fiber; said
first layer being diffusion bonded to said second layer, wherein
said metal is selected from the group consisting of tin, indium and
tin lead alloy.
4. The laminate as recited in claim 1 wherein the superconductor is
selected from the group consisting of YBa.sub.2 Cu.sub.3
O.sub.7-.delta., Bi.sub.2 Sr.sub.3-x Ca.sub.x Cu.sub.2 O.sub.8+y,
and Tl.sub.2 Ba.sub.2 CaCu.sub.2 O.sub..delta..
5. The laminate as recited in claim 2 wherein the superconductor is
selected from the group consisting of YBa.sub.2 Cu.sub.3
O.sub.7-.delta., Bi.sub.2 Sr.sub.3-x Ca.sub.x Cu.sub.2 O.sub.8+y,
and Tl.sub.2 Ba.sub.2 CaCu.sub.2 O.sub..delta..
6. The laminate as recited in claim 4 wherein the superconductor is
YBa.sub.2 Cu.sub.3 O.sub.7-.delta..
7. The laminate as recited in claim 5 wherein the superconductor is
YBa.sub.2 Cu.sub.3 O.sub.7-.delta..
8. The laminate as recited in claim wherein the metal is selected
from the group consisting of tin, indium and tin lead alloy.
9. The laminate as recited in claim 8 wherein the metal is tin.
10. The laminate as recited in claim 2 wherein the metal is
selected from the group consisting of tin, indium and tin lead
alloy.
11. The laminate as recited in claim 6 wherein the metal is
selected from the group consisting of tin, indium and tin lead
alloy.
12. The laminate as recited in claim 7 wherein the metal is
selected from the group consisting of tin, indium and tin lead
alloy.
13. The laminate as recited in claim 10 wherein the second and
third layers comprise the same metal.
14. The laminate as recited in claim 10 wherein the second and
third layers comprise different metals.
15. The laminate as recited in claim 13 wherein the metal is
tin.
16. The laminate as recited in claim 14 wherein the metals are tin
and tin lead alloy.
17. The laminate as recited in claim 1 wherein the second layer
further comprises carbon fibers.
18. The laminate as recited in claim 2 wherein the second layer
further comprises carbon fibers.
19. The laminate as recited in claim 6 wherein the second layer
further comprises carbon fibers.
20. The laminate as recited in claim 7 wherein the carbon fibers
are from textile fiber precursors.
21. The laminate as recited in claim 17 wherein the carbon fibers
are from textile fiber precursors.
22. The laminate as recited in claim 18 wherein the carbon fibers
are from textile fiber precursors.
23. The laminate as recited in claim 19 wherein the carbon fibers
are from textile fiber precursors.
24. The laminate as recited in claim 21 wherein the textile fibers
are PAN based.
25. The laminate as recited in claim 22 wherein the textile fibers
are PAN based.
26. The laminate as recited in claim 1 wherein the metal will be
able to diffuse into the surface of the superconducting layer at a
temperature lower than the degradation temperature of the
superconductor.
27. The laminate as recited in claim 2 wherein the metal will be
able to diffuse into the surface of the superconducting layer at a
temperature lower than the degradation temperature of the
superconductor.
28. The laminate as recited in claim 1 wherein the superconductor
is a superconducting composite.
29. The laminate as recited in claim 2 wherein the superconductor
is a superconducting composite.
30. The laminate as recited in claim 28 wherein the superconducting
composite comprises a metal.
31. The laminate as recited in claim 29 wherein the superconducting
composite comprises a metal.
32. The laminate as recited in claim 30 wherein said
superconducting composite is fabricated by powder metallurgy.
33. The laminate as recited in claim 31 wherein said
superconducting composite is fabricated by metal tube drawing.
34. The laminate as recited in claim 1 further comprising a
plurality of alternating superconducting and metal containing
layers.
35. The laminate as recited in claim 1 wherein the first layer
comprises between about 20 to about 60 vol. % of the laminate.
36. The laminate as recited in claim 2 wherein the first layer
comprises between about 20 to about 60 vol. % of the laminate.
37. The laminate as recited in claim 1 wherein the second layer
comprises between about 40 to about 80 vol. % of the laminate.
38. The laminate as recited in claim 17 wherein the carbon fibers
comprise between about 15-40 vol. % of said second layer.
39. The laminate as recited in claim 1 wherein the second layer is
between about 0.1 mm to about 6 mm in thickness.
40. The laminate as recited in claim 35 wherein the second layer is
between about 0.1 mm to about 6 mm in thickness.
41. The laminate as recited in claim 36 wherein the second layer is
between about 0.1 mm to about 6 mm in thickness.
42. The laminate as recited in claim 38 wherein the second layer is
between about 0.4 mm to about 6 mm in thickness.
43. The laminate as recited in claim 1 wherein the first layer is
at least 1000 Angstroms in thickness.
44. The laminate as recited in claim 2 wherein the first layer is
at least 1000 Angstroms in thickness.
45. The laminate as recited in claim 40 wherein the first layer is
at least 1000 Angstroms in thickness.
46. The laminate as recited in claim 42 wherein the first layer is
at least 1000 Angstroms in thickness.
47. A superconducting laminate comprising, first and second layers;
said first layer comprising about 40 vol. % YBa.sub.2 Cu.sub.3
O.sub.7-.delta., and being about 3.4 mm thick; said second layer
comprising about 60 vol. % of tin and being about 2.3 mm thick;
said first layer being diffusion bonded to said second layer at
140.degree. C. and 5.3 MPa for about 15 minutes.
48. A superconducting laminate prepared by a process comprising the
steps of:
a) preparing a high T.sub.c superconducting layer;
b) preparing a metal layer; and
c) diffusion bonding said superconducting layer to said metal
layer;
wherein said metal is selected from the group consisting
essentially of tin, indium and tin lead alloy.
Description
BACKGROUND OF THE INVENTION
The present invention relates to superconductor-metal laminates and
to a method of making such laminates.
The high T.sub.c (superconducting transition temperature)
superconductors are generally brittle, hard to shape, and high in
electrical resistivity at normal temperatures. By high T.sub.c
superconductors are meant superconductors with T.sub.c above the
temperature of liquid nitrogen (77.degree. K.). Typically, metals
are not high T.sub.c superconductors and at normal temperatures are
ductile, formable, low in electrical resistivity and high in
thermal conductivity.
It has been recognized that the combination of a high T.sub.c
superconductor and a metal in the form of a composite material is
attractive for the following reasons: the combination has improved
toughness, ductility, shapeability and formability; improved
electrical and thermal stabilization; and improved critical current
density. The critical current density is the current density above
which the superconductor loses its superconductivity, at a given
temperature below T.sub.c.
Powder metallurgy has been used to fabricate superconductor-metal
composites. (In-Gann Chen, S. Sen and D. M. Stefanescu, Appl. Phys.
Lett. 52 (16), 1355 (1988); F. H. Streitz, M. Z. Cieplak, Gang
Xiao, A. Gavrin, A. Bakhshai and C. L. Chien, Appl. Phys. Lett. 52,
927 (1988); A. Goyal, P. D. Funkenbusch, G. C. S. Chang and S. J.
Burns, Mater. Lett. 6 (8-9), 257 (1988)). Generally powder
metallurgy involves mixing superconductor powder and metal powder,
followed by sintering the mixture. There are some problems with
this method. For example, the metal content is limited to 50 vol. %
or below in order to have a continuous superconducting path in the
composite. This limits the ductility of the composite. In addition,
the choice of metal is limited to metals that are stable at the
sintering temperature in oxygen and do not react with the
superconductor at the sintering temperature (typically 950.degree.
C. for YBa.sub.2 Cu.sub.3 O.sub.7-.delta.).
Another method that has been used in forming a superconductor-metal
composite involves (i) packing a superconductor powder in a metal
tube, (ii) drawing the tube to a smaller diameter, and (iii)
sintering. This method is commonly known as the metal tube drawing
method. (R. W. McCallum, J. D. Verhoeven, M. A. Noack, E. D.
Gibson, F. C. Laabs and D. K. Finnemore, Advanced Ceramic Materials
2 (3B), 388 (1987); S. Jin, R. C. Sherwood, R. B. Van Dover, T. H.
Tiefel and D. W. Johnson, Jr., Appl. Phys. Lett. 51, 203 (1987)).
Similar drawbacks as with the powder metallurgy method have been
recognized with this method. For example, the choice of metal is
limited to metals that are stable at the high sintering temperature
required by the superconductor.
A further disadvantage of the superconductor-metal composites
formed by the above described methods is that they are generally
weak in tension.
It is therefore an object of this invention to provide a method for
fabricating superconductor-metal materials which overcome the
problems recognized in the art.
It is a further object to provide a method of producing a
superconductor-metal material which overcomes the limitations of
known methods while retaining the desirable characteristics.
A still further object is to produce a superconductor-metal
material having both good tensile and compressive strength.
SUMMARY OF THE INVENTION
The invention is a superconducting laminate comprising, first and
second layers; said first layer comprising a ceramic superconductor
and said second layer comprising a metal; said first layer being
diffusion bonded to said second layer.
The invention further comprises a superconducting laminate
comprising first and second layers; said first layer comprising a
ceramic superconductor and said second layer comprising a metal and
carbon fibers; said first layer being bonded to said second
layer.
The invention also comprises a method for preparing a
superconducting laminate comprising the steps of, a) preparing a
superconducting layer; b) preparing a metal layer; and c) diffusion
bonding said superconducting layer with said metal layer.
The invention includes a method for preparing a superconducting
laminate comprising the steps of, a) preparing a superconducting
layer; b) preparing a metal-carbon fiber layer; and c) bonding said
superconducting layer with said metal-carbon fiber layer.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 Is an SEM (Scanning Electron Microscope) photograph of a
portion of a polished section of an MMC-YBa.sub.2 Cu.sub.3
O.sub.7-.delta. -MMC (MMC is the metal-matrix carbon fiber
composite) three-layer laminate containing 32.1 vol. % fiber and
50.2 vol. % Sn (Tin). The left half is the MMC, the right half is
the superconductor. The small dumbbell shaped spots in the MMC are
the tips of carbon fibers.
FIG. 2 Shows the dependence of the electrical resistivity on
temperature for the plain superconductor (solid circles), for a
laminate containing 80.1% vol. % Sn and no fibers (open circles),
and for a laminate containing 32.1 vol. % fibers and 50.2 vol. % Sn
(crosses).
FIG. 3 Shows the compressive stress-strain curves (up to fracture)
for a laminate without fibers (24 vol. % Sn) (open circles) and a
laminate with fibers (51.1 vol. % Sn and 26.2 vol. % fibers) (solid
circles).
FIG. 4 Illustrates the effect of tin content on the compressive
ductility of laminate without carbon fibers; error bar of
.+-.7%.
FIG. 5 Shows the effects of tin and fiber contents on the
compressive ductility; error bar of .+-.5%.
FIG. 6 Is an SEM photograph of the compressive fracture surface of
a laminate containing 19.5 vol. % Sn and no fibers. The right
one-third of the photograph is tin, the left two-thirds of the
photograph is the superconductor.
FIG. 7 Shows the effects of tin and fiber contents on the
compressive elastic modulus; error bar of .+-.8%.
FIG. 8 Shows the effects of tin and fiber contents on the
compressive strength; error bar of .+-.9%.
FIG. 9 Shows the tensile stress-strain curves (up to fracture) for
a laminate without fibers (24 vol. % Sn) (open circles) and a
laminate with fibers (51.4 vol. % Sn and 27.2 vol. % fibers) (solid
circles).
FIG. 10 Illustrates the effects of tin and fiber contents on the
tensile strength; error bar of .+-.8%.
FIG. 11 Illustrates the effects of tin and fiber contents on the
tensile ductility; error bar of .+-.5%.
FIG. 12 Shows the effects of tin and fiber contents on the tensile
modulus; error bar of .+-.7%.
FIG. 13 Shows the dependence of the tensile strength on the carbon
fiber content in vol. % fibers, (the volume of the superconductor
is not counted in calculating the volume % fibers). Circles:
without superconductor (MMC alone), error bar of .+-.9%; crosses:
with superconductor, error bar of .+-.9%.
FIG. 14 Depicts the dependence of the tensile modulus on the carbon
fiber content in vol. % fibers, (the volume of the superconductor
is not counted in calculating the volume % fibers). Circles:
without superconductor (MMC alone), error bar of .+-.8%; crosses:
with superconductor, error bar of .+-.7%.
FIG. 15 Shows the dependence of the tensile ductility on the carbon
fiber content in vol. % fibers, (the volume of the superconductor
is not counted in calculating the volume % fibers). Circles:
without superconductor (MMC alone), error bar of .+-.6%; crosses:
with superconductor, error bar of .+-.5%.
FIG. 16 Shows the dependence of the electrical resistivity on
temperature for a laminate, containing 50.2 vol. % Sn and 31.0 vol.
% fibers, that has been subjected to tension up to a stress of 90.6
MPa and a strain of 0.91% .
DETAILED DESCRIPTION OF THE INVENTION
By superconducting laminate is meant an article having at least 2
layers, at least one of which comprises a high T.sub.c
superconductor and at least one of which comprises a metal.
A metal as used herein means any metal which will be able to
diffuse into the surface of the superconducting layer at a
temperature lower than the degradation temperature of the
superconducting material, and includes alloys. Examples of suitable
metals include, but are not limited to tin, indium, and tin-lead
alloy. A preferred metal is tin (Sn). The metal may be in a form
suitable for forming the laminates of the present invention and
includes foils, ingots, sheets, and powders. The thickness of the
metal layer may vary in accordance with the invention, preferably,
the thickness of the metal where no carbon fibers are incorporated
is between about 0.1 mm to about 6 mm. The thickness of the metal
layer where carbon fibers are incorporated is between about 0.4 mm
to about 6 mm.
In accordance with this invention, the laminates may comprise
between about 1 to about 99 vol. % metal (total volume being the
volume of metal and the volume of superconductor). Preferred
laminates comprise between about 40 to about 80 vol. % metal. Most
preferred laminates comprise about 80 vol. % metal. Although the
preferred and most preferred laminates are discussed above, the
actual vol. % of the metal utilized will also depend on the
thickness of the superconductor layer.
Superconductor as used herein is any ceramic superconductor and may
include superconductor composites. Examples of superconductors are
YBa.sub.2 Cu.sub.3 O.sub.7-.delta., Bi.sub.2 Sr.sub.3-x Ca.sub.x
Cu.sub.2 O.sub.8+y, and Tl.sub.2 Ba.sub.2 CaCu.sub.2 O.sub..delta..
A preferred superconductor is YBa.sub.2 Cu.sub.3 O.sub.7-.delta..
The superconductor can be in bulk, wire or film form as prepared by
any known technique. For example, for the superconductor YBa.sub.2
Cu.sub.3 O.sub.7-.delta., the YBa.sub.2 Cu.sub.3 O.sub.7-.delta.
powder (commercially available from a variety of sources known to
those skilled in the art) was pressed at a pressure of 137 MPa
(mega pascal) and then sintered at 950.degree. C. for 12 h,
followed by annealing at 420.degree. C. in flowing oxygen for 1 h.
Superconductor composites include composites which comprise
superconductor and metal and includes those which may be formed by
the powder metallurgy method, the metal tube drawing method or
other known methods. Generally, laminates having a superconducting
layer in the order of at least 1000 angstroms in thickness are
preferred.
The laminates of the present invention have a continuous
superconducting path and the superconducting layer retains it's
high T.sub.c superconducting property after incorporation into the
laminate.
While various lamination methods may be used, the laminates are
preferably formed by diffusion bonding, e.g. by hot pressing the
superconducting material and the metal, in air at a temperature
lower than the melting point of the metal (which melting point, in
accordance with this invention, is lower than the degradation
temperature of the superconductor). Diffusion bonding as used
herein means the solid state bonding of two materials by using heat
and pressure. The degradation temperature of the superconducting
material means the temperature above which the material no longer
is a high T.sub.c superconductor. For example, for the
superconductor YBa.sub.2 Cu.sub.3 O.sub.7-.delta., the degradation
temperature is about 300.degree. C., and hot pressing would be
conducted at a temperature lower than 300.degree. C.
Hot pressing is for a sufficient time and at a sufficient pressure
for diffusion bonding to occur. For a given metal, the temperature,
pressure and time may be adjusted so that various combinations of
temperature, pressure and time may be utilized to achieve the
diffusion bond. As is well known in the art, typically, if a higher
temperature is used, a lower pressure and shorter time may be
sufficient. The pressure utilized, however, is limited by the
compressive strength of the superconductor.
In another embodiment of the invention, a laminate having three
layers may be prepared. The superconductor may have a metal layer
on either side of the superconductor to package the superconductor.
This laminate may be prepared as described above by hot pressing a
layer of superconductor sandwiched by two layers of metal, one on
either side of the superconductor at a temperature below the
melting point of the metal at a sufficient pressure and for a
sufficient time in order to achieve diffusion bonding.
It is within the scope of this embodiment that the two metal layers
on either side of the superconductor layer may comprise the same or
different metals. More particularly, both layers may be tin or one
metal layer may be tin and the other metal layer may be a tin-lead
alloy or indium or any other combination, which would meet the
above discussed parameters.
It is also within the scope of this invention to have a plurality
of ceramic superconductor layers alternating with metal layers.
In another embodiment of the invention, a reinforced laminate
having greater tensile strength was made by the addition of carbon
fibers to the metal layer. The carbon fiber reinforced metal
provides an effective way of packaging ceramic superconductors so
that the package exhibits good mechanical, electrical and thermal
properties. The carbon fibers may include those made from a variety
of precursors such as textile fibers, pitch or other precursors. An
example of a textile fiber is PAN (polyacrylonitrile).
The fibers may be in a continuous or short form. If the continuous
form is used, they may be unidirectional, multidirectional or in
the form of a woven fabric. In a preferred embodiment, continuous,
unidirectional carbon fibers may be utilized. Between about 0 vol.
% to about 50 vol. % of carbon fiber (the volume being the volume
of metal and carbon fiber) may be incorporated into the metal
layer. It is preferred that about 15-40 vol. % of carbon fiber is
incorporated into the metal layer.
The carbon fiber reinforced metal-superconductor laminate may be
prepared by using a two-step process. The first step is the
preparation of a metal-matrix carbon fiber composite (abbreviated
MMC) by laying up carbon fibers and metal in the form of alternate
layers and consolidating by hot pressing (by using a hydraulic
press or other known methods) at a temperature, pressure and time
which will allow the metal to penetrate or diffuse into void spaces
between the carbon fibers. The second step involves hot pressing a
layer of superconductor sandwiched by two layers of MMC at a
sufficient temperature, preferably below below the melting point of
the metal, and at a sufficient pressure for a sufficient time in
order to achieve bonding between the MMC and the superconductor.
Bonding means the liquid or solid state joining of two or more
materials. Methods of bonding, as used herein, includes adhesive
bonding, soldering, brazing, welding and diffusion bonding.
It is to be understood, particularly in the case of carbon fiber
containing materials, that lamination or bonding methods other than
diffusion bonding may be used. Such methods, however, may have
serious disadvantages related to physical limitations of certain
materials which may allow degradation or destruction of such
materials, as a result of the conditions encountered during
lamination.
EXAMPLES
EXAMPLE 1
To make a superconductor-tin laminate (without carbon fibers), in
accordance with this invention, the superconductor YBa.sub.2
Cu.sub.3 O.sub.7-.delta. was prepared as described above, tin foil
(60 vol. %, 4.12 g, 3.36 mm thick, includes both tin layers) was
diffusion bonded to both sides of the YBa.sub.2 Cu.sub.3
O.sub.7-.delta. superconductor layer (40 vol. %, 1.54 g, 2.24 mm
thick) by hot pressing in air at 140.degree. C. and 5.3 MPa for 15
minutes. Table 1 below lists other percentages of tin which were
used and the various thicknesses of the tin (includes both layers)
and the superconductor.
TABLE 1 ______________________________________ Thickness (mm) Vol.
% Sn Superconductor Tin ______________________________________ 81.6
0.460 2.032 77.8 0.580 2.032 73.4 0.741 2.032 64.5 1.126 2.032 50.1
2.036 2.032 36.0 1.803 1.016 33.3 2.032 1.016 29.4 2.448 1.016 26.5
2.821 1.016 25.0 3.058 1.016 24.0 3.213 1.016 22.2 3.556 1.016 20.0
4.063 1.016 19.5 1.676 0.406 15.6 1.372 0.254 10.7 2.381 0.254 6.1
3.897 0.254 ______________________________________
EXAMPLE 2
To produce a carbon fiber reinforced tin-superconductor laminate in
accordance with this invention, the previously described two-step
process may be utilized.
In the first step, a tin-matrix unidirectional carbon fiber
composite (abbreviated MMC), was prepared by laying up carbon
fibers (15 vol. %, 0.285 g; continuous, unidirectional, and
PAN-based, which are commercially available) and tin foils (50 vol.
%, 3.75 g) in the form of alternate layers and consolidating by hot
pressing (by using a hydraulic press) at 244.degree. C. and at a
pressure of 64 MPa for 20 min. The thickness of the MMC layer was
4.06 mm. In the second step a layer of superconductor YBa.sub.2
Cu.sub.3 O.sub.7-.delta. (35 vol. %. 1.21 g, 2.18 mm thick)
sandwiched by two layers of MMC was hot pressed at 180.degree. C.
and 5.1 MPa for 15 min in order to achieve diffusion bonding. Note
that 170.degree. C. is below the melting point of tin (232.degree.
C.).
TEST RESULTS OF THE LAMINATES
In the resulting laminate, tin served as the adhesive and to
increase the ductility, the normal-state electrical conductivity
and the thermal conductivity. Carbon fibers served to increase the
strength and the modulus, both in tension along the fiber direction
and in compression perpendicular to the fiber layers, and also
served to increase the thermal conductivity and the thermal fatigue
resistance, as shown in table 2 below. Volume % as used in Table 2
includes the volume of carbon, metal and superconductor.
TABLE 2 ______________________________________ Thermal fatigue due
to cycling between room temperature and liquid nitrogen temperature
Carbon fiber Content Sn content No. of cycles for (vol. %) (vol. %)
delamination to start* ______________________________________ 0
43.2 63 3.0 39.4 86 8.3 40.1 102 12.0 43.1 116 15.3 48.3 123 20.1
50.2 127 ______________________________________ *Each number is the
average of three specimens.
Laminates of various fiber contents were subjected to thermal
cycling between room temperature and liquid nitrogen temperature
(77.degree. K.) by immersion in liquid nitrogen for 20 min,
followed by room temperature equilibration for at least 30 min, and
repeating. After every cycle, each specimen was observed under an
optical microscope to look for delamination (slight cracking)
between the superconductor and the MMC. Table 2 shows the number of
cycles for the start of delamination for each laminate composition.
The higher the carbon fiber content, the greater the number of
cycles required for the start of delamination.
The fabrication of the laminates, according to this invention
involves relatively low temperatures. The simplicity of this
process makes it possible for an operation to be set up for
fabricating continuous superconducting cables which are both
shielded and toughened by metal and strengthened by carbon
fibers.
The toughness was measured by carrying out the Izod Test using a
Tinius Olsen plastic impact tester. The toughness was 3.25 in.lb
for a laminate containing 18.8 vol. % Sn (no fibers) and was 5.50
in.lb for a laminate containing 25.5 vol. % Sn (no fibers). Hence,
the tin greatly enhanced the toughness.
As can be seen from the SEM photograph shown in FIG. 1, no void or
crack was observed between the MMC and the superconductor or
between the tin and the carbon fibers in the MMC.
Electrical resistivity of the laminates can be seen in FIG. 2. The
electrical resistivity was measured with the four-probe technique
by using a Keithley 181 nanovolt meter and a Keithley 224
programmable current source, such that the current was around
10.sup.-3 amperes. A thermocouple was placed so that it almost
touched the sample. In the case of laminates containing fibers, the
electrical resistivity was measured in the direction of the fibers
however, the electrical resistivity in the direction perpendicular
to the fiber axis was comparable to that in the direction parallel
to the fibers. The drop to zero resistivity is slightly less sharp
for either laminate than the plain superconductor. Carbon fibers
have a lower electrical resistivity than the plain superconductor
above T.sub.c, but its value is higher than that of tin. Therefore,
the normal-state electrical resistivity of the laminate with tin
and fibers is lower than that of the plain superconductor and
higher than that of the laminate with tin and no fiber.
Mechanical testing was performed using a hydraulic Materials
Testing Systems (MTS). The strain in tensile or compressive testing
was measured by the displacement of the crosshead. The gage length
was 34.7 mm for tensile testing and 6 mm for compressive testing.
Compressive testing was performed with the force perpendicular to
the laminate layers. Tensile testing was performed with the force
parallel to the fibers in the plane of the laminate. At least three
samples were run and the data were averaged for each composition in
each type of test.
FIG. 3 shows the compressive stress-strain curves (up to fracture)
for a laminate containing tin (24 vol. % Sn) but no fibers (open
circles) and a laminate containing 51.1 vol. % Sn and 26.2 vol. %
fibers (solid circles).
FIG. 4 shows that tin greatly increases the ductility of the
laminate. FIG. 5 shows that tin greatly increased the ductility of
the laminate, but the fibers decreased the ductility.
As can be seen in FIG. 6, cracks are observed to extend from the
tin superconductor interface inward into the superconductor. A
similar fracture is observed in a laminate with both fibers and
tin. In contrast, the plain superconductor (without tin or fiber)
shatters and disintegrates into particles upon fracture. Hence, the
fracture behavior is dramatically different between either laminate
and the plain superconductor.
The decrease of the modulus with increasing tin content and
decreasing fiber content is significant and can be seen in FIG. 7.
No systematic trend can be seen in FIG. 8 in the compressive
strength, because the strength is very sensitive to small flaws
that are bound to be present in the superconductor.
FIG. 9 shows the tensile stress-strain curves (up to fracture) for
a laminate containing tin (24 vol. % Sn) but no fibers (open
circles) and a laminate containing 51.4 vol. % Sn and 27.2 vol. %
fibers (solid circles).
FIGS. 10, 11 and 12 show the effects of tin and fiber contents on
the tensile test results. The tensile test was performed along the
fiber direction. The addition of carbon fibers greatly improved the
tensile strength (FIG. 10), but decreased the ductility slightly
(FIG. 11). The tensile modulus was increased by increasing the
fiber content. Debonding between the MMC and the superconductor and
some fiber pull-out were observed from the fracture surface after
the tensile test.
FIGS. 13, 14 and 15 show the effects of the superconductor on the
tensile test results. Data obtained with the presence of the
superconductor (in the form MMC-superconductor-MMC) are shown by
circles. Data obtained with the absence of the superconductor
(i.e., MMC alone) are shown by crosses. The horizontal scale of
FIGS. 13, 14 and 15 show the vol. % fibers such that the volume of
the superconductor was not counted in the volume of the laminate.
(Note that this scale is different from the horizontal scale of
FIGS. 4, 5, 7, 8, 10, 11 and 12 in which the volume of the
superconductor was counted in calculating the volume percent
fibers.) The similarity of data with and without the superconductor
in FIGS. 13 and 14 shows that the tensile load was almost totally
sustained by the MMC. FIG. 15 shows that the tensile ductility is
somewhat diminished by the presence of the superconductor.
The tensile test for the plain superconductor (without tin or
fibers) did not give very reliable results because of the
difficulty of gripping the superconductor, which was difficult in
the absence of tin. Nevertheless, the tensile strength was about
5.2 MPa, the tensile modulus was roughly 1.3 GPa (Giga pascal), and
the tensile ductility was roughly 0.5%.
A laminate containing 31.0 vol. % fibers and 50.2 vol. % tin was
subjected to tension up to a tensile stress of 90.6 Mpa and a
tensile strain of 0.91% (below the stress or strain required for
fracture) and then the load was released (allowing the strain to
return to essentially zero) and the electrical resistivity was
measured as a function of temperature, as shown in FIG. 16. It
remained superconducting.
Comparison of FIG. 8 and 10 shows that, without fibers, the tensile
strength (parallel to the fiber direction) was much less than the
compressive strength (perpendicular to the fiber layers), but with
about 24 vol. % fibers, the tensile strength was approximately
equal to the compressive strength. With further increase of the
fiber content, the tensile strength exceeded the compressive
strength.
The tensile strength of the plain superconductor was roughly 5.2
MPa. The presence of tin (without fibers) increased the value to
about 20 MPa (FIG. 10). Further addition of carbon fibers
significantly increased the tensile strength, up to 134 MPa for 31
vol. % fibers.
Carbon fibers increased the compressive strength, compressive
modulus, tensile strength and tensile modulus, but they decreased
the compressive ductility and tensile ductility. However, because
tin was also present and tin is a soft metal, the compressive
ductility for the case of 31 vol. % fibers was approximately equal
to that for the plain superconductor (without tin or fiber). For
carbon fiber contents less than 30 vol. % fibers, the compressive
ductility exceeded that of the plain superconductor. In general,
the carbon fibers decreased the tensile ductility (FIG. 11) much
less than the compressive ductility (FIG. 6).
FIG. 16 and 11 show that the superconducting behavior of the
laminates was maintained after tension almost to the point of
tensile fracture.
The fabrication of the laminates involved low temperatures. The
simplicity of this process makes it possible for an operation to be
set up for fabricating continuous superconducting cables which are
both shielded and toughened by tin and strengthened by carbon
fibers. In contrast to powder metallurgy, the diffusion bonding
method allows the metal to be the major phase while still
maintaining a continuous superconducting path in the laminate.
Furthermore, carbon fibers, with their nearly zero thermal
expansion coefficient helps match the thermal expansion
coefficients of the MMC layer and the superconductor layer. This
matching is necessary in order to enhance the durability of the
composite to thermal cycling (i.e. thermal fatigue). In addition,
carbon fibers are excellent in thermal conductivity (both at
ambient and cryogenic temperatures), wear resistant, corrosion
resistant, and are low in electrical resistivity.
Other embodiments of the invention will be apparent to the skilled
in the art from a consideration of this specification or practice
of the invention disclosed herein. It should be understood that
there may be other embodiments which fall within the spirit and
scope of the invention as defined by the following claims.
* * * * *